Multi-phase DC-to-DC converter has been widely used in power supplier circuits. A multi-phase buck converter typically employs a pair of MOSFETs connected in series for each phase as an output stage connected between a high voltage and a low voltage to produce a phase output. To obtain stable and balanced output, the output voltage and phase currents of a converter are sensed and fed back to the control circuit of the converter to produce the suitable control signals to manipulate the MOSFETs of the output stage. To feed back the current of each phase, a current sense apparatus is used to detect the current flowing through the phase, for example a scheme provided by U.S. Pat. No. 6,246,220 issued to Isham et al. producing the current sense signal by use of a current feedback resistor to feed back to the control circuit. Since the control of each phase is achieved by referring to the phase current detected by a current sense apparatus, the accuracy of the current sense apparatus will directly affect the phase balance and performance of the converter. However, the introduced resistors will affect the phase current, and unfortunately, the factors of electronic devices are temperature dependent, especially the resistances or transistors made of semiconductor. The increasing working temperature not only produces signal error but also brings the phase at higher temperature further sharing more currents, and thus leads to be burnt out.
Various conventional current sense apparatus used in synchronous switching mode buck converters are shown in FIG. 1. In FIG. 1A, a sense resistor 76 is introduced to be connected in series between the input voltage VIN and high side MOSFET 72, and the produced voltage drop further produces a current sense signal by an operational amplifier 25. In FIG. 1B, the sense resistor 76 is connected in series between a ground and the low side MOSFET 74, and the operational amplifier 25 detects the voltage drop across the resistor 76 to produce the current sense signal. Both of them introduce the additional sense resistor 76, and thus increase the cost and reduce the system efficiency. In FIG. 1C, the operational amplifier 25 directly detects the voltage drop across the conductive high side MOSFET 72 to produce the current sense signal. In FIG. 1D, the operational amplifier 25 directly detects the voltage drop across the conductive low side MOSFET 74 to produce the current sense signal. Both of them utilize the internal resistance of the MOSFET 72 or 74 as the sense resistor, and thus need not more cost for the sense resistor. However, the internal resistance of MOSFET varies with temperature, and the varied rate is about 5000 ppm, it is therefore not accurate of the measured current sense signal. In FIG. 1E, the parasitic resistor 78 of the output inductor 23 is used as the sense resistor, and it can be treated as connected in series between the inductor 23 and converter output 70. The operational amplifier 25 detects the voltage drop across the parasitic resistor 78 to produce the current sense signal, while the resistance of the parasitic resistor 78 is too small and hard to control. In FIG. 1F, the sense resistor 76 is connected in series between the inductor 23 and converter output 70, and the operational amplifier 25 detects the voltage drop across the resistor 76 to produce the current sense signal. This method introduces an additional resistor, and hence higher cost and poor system efficiency.
FIG. 6 shows a converter employing a conventional current sense apparatus as that in FIG. 1D, and only one phase is shown for simplicity. The operational amplifier 25 detects the voltage drop across the conductive low side MOSFET 74 and the produced current sense signal is connected to a sampling/holding circuit 50 that is also connected to the non-inverting input 302 of the error amplifier 30. Additionally, a voltage follower 32 connected with an original reference voltage REF produces a reference voltage to the node between resistor 34 and capacitor 36. The other terminal of the resistor 34 is connected to the non-inverting input 302 of the error amplifier 30. The inverting input 301 of the error amplifier 30 is connected with the output voltage VOUT, and a feedback signal 303 and the output of the sampling/holding circuit 50 are connected to the control logic 40 together to manipulate the output stage circuit, i.e., MOSFETs 72 and 74. Due to the current sense signal relating to the internal resistance of the MOSFET 74, which is temperature dependent, the current sense signal will change with temperature and result in error. Moreover, the converter output varies when load 60 changes, as shown in FIG. 7. FIG. 7A shows the waveforms of the converter output at low temperature, of which the upper one shows the transient performance of the variation ΔIout of the converter output current Iout resulted from load variation, and the lower one shows the ripple performance of the converter output voltage VOUT induced by this transient effect. FIG. 7B shows the waveforms of the converter output at high temperature. For the same load variation, the droop VDROOP of the converter output voltage VOUT is smaller at high temperature than that at low temperature. In other words, the performance of a converter is much affected by temperature. FIG. 8 shows a curve of the internal resistance of MOSFET to temperature variation. When temperature rises, the internal resistance of MOSFET also becomes larger, and therefore all operations incorporating the utilization of the internal resistance of MOSFET are affected by temperature.